In-ear monitors, also referred to as IEMs, canal phones and stereo earphones, are commonly used to listen to both recorded and live music. A typical recorded music application would involve plugging a pair of monitors into a music player such as a CD player, flash or hard drive based MP3 player, home stereo or similar device using the device's headphone socket. Alternately, the monitors can be wirelessly coupled to the music player. In a typical live music application, the on-stage musician uses the monitors in order to hear his or her own music during a performance. In this case, the monitor is either plugged into a wireless belt pack receiver or directly connected to an audio distribution device such as a mixer or a headphone amplifier. This type of monitor offers numerous advantages over the use of stage loudspeakers, including improved gain-before-feedback, minimization/elimination of room/stage acoustic effects, cleaner mix through the minimization of stage noise, increased mobility for the musician and the reduction of ambient sounds. Many of these same advantages may be gained by an audience member using in-ear monitors to listen to a live performance.
In-ear monitors are quite small and are normally worn just outside the ear canal. As a result, the acoustic design of the monitor must lend itself to a very compact design utilizing small components. Some monitors are custom fit (i.e., custom molded) while others use a generic “one-size-fits-all” earpiece. Generic earpieces may include a removable and replaceable eartip sleeve that provides a limited degree of customization, e.g., choice of color, size, material and shape.
Prior art in-ear monitors use either diaphragm-based receivers, armature-based receivers, or a combination of the two. Broadly characterized, a diaphragm is a moving-coil speaker with a paper or mylar diaphragm. Since the cost to manufacture a diaphragm is relatively low, they are widely used in many common audio products (e.g., ear buds). In contrast to the diaphragm approach, an armature receiver utilizes a piston design. Due to the inherent cost of armature receivers, however, they are typically only found in hearing aids and high-end in-ear monitors.
Diaphragm receivers, due to the use of moving-coil speakers, suffer from several limitations. First, because of the size of the diaphragm assembly, a typical earpiece is limited to a single diaphragm. This limitation precludes achieving optimal frequency response (i.e., a flat or neutral response) through the inclusion of multiple diaphragms. Second, diaphragm-based monitors have significant frequency roll off above 4 kHz. As the desired upper limit for the frequency response of a high-fidelity monitor is at least 15 kHz, diaphragm-based monitors cannot achieve the desired upper frequency response while still providing accurate low frequency response.
Armatures, also referred to as balanced armatures, were originally developed by the hearing aid industry. This type of driver uses a magnetically balanced shaft or armature within a small, typically rectangular, enclosure. As a result of this design, armature drivers are not reliant on the size and shape of the enclosure, i.e., the ear canal, for tuning as is the case with diaphragm-based monitors. Typically, the length of tubing attached to the armature in combination with an acoustic filter is used to tune the armature. A single armature is capable of accurately reproducing low-frequency audio or high-frequency audio, but incapable of providing high-fidelity performance across all frequencies.
To overcome the limitations associated with both diaphragm and armature drivers, some in-ear monitors use either a combination of both diaphragm and armature drivers or multiple armatures. In such a multi-driver arrangement, a crossover network is used to divide the frequency spectrum into multiple regions, i.e., low and high or low, medium, and high. Separate, optimized drivers are then used for each acoustic region. Generally either a single delivery tube or a pair of delivery tubes delivers the sound produced by the drivers to the output face of the earpiece.
As briefly described above, a variety of techniques are typically used to tune driver output as well as achieve the desired IEM acoustic performance for a specific set of IEMs, these techniques including optimization of driver placement, tubing diameter and length, damper/filter selection, and port placement and size. In general, these techniques are integrated into the fabrication process used to manufacture a pair of molded in-ear monitors. While these techniques may be used to successfully achieve the desired performance, due to the labor intensive nature of these processes both IEM cost and manufacturing time are dramatically affected. Accordingly, what is needed is an IEM manufacturing technique that reduces fabrication complexity while still achieving the requisite acoustic performance. The present invention provides such an IEM manufacturing technique.